1. Field of the Invention
This invention generally relates to automatic exposure control systems for use in photographic camera apparatus, and, more particularly, to an arrangement between a single photodetector and an electro-mechanical shutter which cooperate in a pre-exposure mode to detect the presence of a low scene brightness condition, and in an exposure mode, to control the exposure delivered to a photosensitive film.
2. Description of the Prior Art
It is generally well known among photographers that the light reflected from a scene to be photographed at times is insufficient to take sharp, acceptably exposed pictures without using a source of artificial illumination such as a photoflash lamp or strobe to supplement the available natural light.
Determining exactly when the available natural light is insufficient to provide a proper level of scene illumination, however, is a fairly complex process which involves a consideration of such factors as film speed, camera exposure delivery capability, and the shutter speed at which camera motion is likely to cause blurred pictures.
Those skilled in the photographic arts have recognized the problems associated with determining when there is adequate natural light and have provided well known apparatus (light meters) for use with cameras having manual shutters by which a photographer can measure the scene brightness and quickly correlate the relationships between the important factors and the measured scene brightness to determine if an auxiliary light source is necessary.
In the case of cameras having automatic exposure systems where the photographer has little control over shutter speed or f-number or both, and, probably does not know what the shutter program is for any scene brightness, those skilled in the art have provided apparatus by which the photographer is automatically alerted of the presence of a scene brightness condition which will cause exposure problems.
Examples of the latter type of apparatus are well known in the patent literature. For instance, U.S. Pat. No. 3,611,892 issued to Wolfgang Ort et. al. on Oct. 12, 1971 and entitled "Device for Indicating Underexposure in Photographic Cameras with Automatic Exposure Control", discloses a device for automatically indicating an underexposure condition and setting a diaphragm aperture in response to the movement of a shutter release member which is depressed by the photographer. This device, however, requires the use of a separate fixed-speed shutter which is not automatically controlled by a photocell to provide the correct exposure interval during which the diaphragm aperture is opened.
Another example is described in U.S. Pat. No. 3,855,601 issued to Takashi Uchiyama et. al. where the patentees describe a camera which has a built-in low light level indicator in the sense that a changeover means will operate a display when the output of a comparator is such as to indicate that flash illumination is required. Two photocells are used here which do not appear to be associated with the camera shutter.
A further example is described in U.S. Pat. No. 3,928,859 issued to Dean M. Peterson on Dec. 23, 1975 and entitled "Shutter-Iris for Automatic Camera". Here the patentee describes an automatic camera which generates an anticipatory signal about 3 milliseconds after the start of an exposure interval. If sufficient light is present, a flash circuit is deactivated; if not, the flash is activated. The photocell of the camera is positioned to respond to light reflected from the film. Light from the scene being photographed is not applied to the photocell prior to the incidence of light onto the film and thus there is no preexposure indication to warn the photographer of a low scene brightness condition.
In U.S. Pat. No. 4,007,469 issued to Edwin H. Land et. al. on Feb. 8, 1977, entitled "Photographic Apparatus with Plurality of Selectively Determinable Operational Modes", the patentees describe a camera which includes a counter-reciprocating shutter blade arrangement, two photocells, and associated circuitry which, among other things, provide a visual display in the camera's viewfinder indicating the presence of a low scene brightness condition and the absence of a fully charged strobe condition.
Thus low light level indicators have heretofore either failed to provide a pre-exposure warning to indicate the presence of a low scene brightness prior to exposure or utilized at least two photocells in order to provide the pre-exposure warning with one of the photocells while controlling the actual film exposure by way of the other photocell.
Therefore it is a primary object of this invention to provide an automatic exposure control system having a scanning type, counter-reciprocating shutter blade and a single photodetector whose output is initially used in a pre-exposure mode to detect and indicate the presence of a low scene brightness condition and, thereafter, is used in an exposure mode to control the exposure delivered to the film.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, the combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure.